Interferometric phase-to-intensity noise is generated in optical fiber communication systems when a small spurious delayed secondary fraction of the optical signal is combined with the primary optical signal at the receiver. See James L. Gimlett and Nim K. Cheung, J. Lightwave Technology, Vol. 7, No. 6, June 1989, 888-895. Because of phase fluctuations of the laser source, the interference of these two waves is time-dependent, producing intensity noise on the signal.
For example, when two small reflectances R.sub.1 and R.sub.2 at points along an optical fiber allow signal light to be doubly reflected between them, the doubly-reflected light is time-delayed and continues in the same direction as the direct light. The original signal and the doubly-reflected light are thus capable of interfering with one another to produce intensity noise. This noise contribution is especially strong when the delay time is greater than the coherence time of the source laser. The effect is further exacerbated in systems containing optical amplifiers when a gain element lies between the reflectances. See Hisao Yoshinaga, Koji Kikushima, and Etsugo Yoneda, J. Lightwave Technology, Vol. 10, No. 8, Aug. 1992, 1132-1136. This is especially so when the secondary signal is amplified more than the primary signal.
In both analog and digital optical systems, acceptable limits on optical isolation and reflections from connectors and other components are dictated by this interferometric phase-to-intensity noise effect. See Gimlett et al., supra; Yoshinaga et al., supra; W. I. Way, C. Lin, C. E. Zah, L. Curtis, R. Spicer, and W. C. Young, IEEE Photonics Technology Lett., Vol. 2, No. 5, May 1990, 360-362; Daniel A. Fishman, Donald G. Duff, and Jonathan A. Nagel, J. Lightwave Technology, Vol. 8, No. 6, June 1990, 894-905; and M. Kobayashi, T. Ishihara, and M. Gotoh, IEEE Photonics Technology Lett., Vol. 5, No. 8, Aug. 1993, 925-928.
The magnitude and electrical frequency dependence of this noise depend upon the relative secondary path intensities (.alpha..sub.I), the time delay, and the optical spectrum of the source. Quantitatively, for cases where the laser source has a Lorentzian line shape and where the time delay of the secondary path exceeds the source coherence time, the frequency-dependent contribution to the noise factor caused by interferometric phase-to-intensity noise (F.sub.INT (f)) is given by: ##EQU1## where f is the electrical frequency at which the noise measurement is made, .nu. is optical frequency of the source, .DELTA..nu. is the optical linewidth of the source, h is Planck's constant, and P.sub.i is the optical input power to the secondary path. See Gimlett et al., supra, and Yoshinaga et al., supra.
For an optical circuit of the type shown in FIG. 1, EQU .alpha..sub.I =R.sup.2.sub.EFF =G.sub.f G.sub.b R.sub.1 R.sub.2 ( 2)
where R.sub.1 and R.sub.2 are effective optical power reflectances, R.sub.1 representing the combined effect of all reflectors located before the gain fiber G and R.sub.2 representing the combined effect of all reflectors located after the gain fiber. For example, R.sub.2 includes the localized reflectance R.sub.2 and the distributed gain-fiber Rayleigh backscattering (RBS) equivalent reflectance R.sub.RBS, which conceptually is shown as being located after the gain fiber in FIG. 1. Thus, EQU R.sub.2 =R.sub.2 '+R.sub.RBS ( 3)
Normally, for steady state conditions, the forward gain G.sub.f and the backward gain G.sub.b are the same, but these gains could be significantly different if, for example, an optical isolator were located between R.sub.1 and R.sub.2.
While it might be possible to measure all of the relative secondary path intensities (.alpha..sub.I) and delay times within an optical amplifier unit (or other optical unit) during construction, it is desirable to determine .alpha..sub.I by means of external measurements on the amplifier considered as a unit. Once this .alpha..sub.I is known, it becomes possible to calculate the interferometric noise which would be produced in specific systems employing the amplifier. The present invention is directed to providing such an external measurement of .alpha..sub.I for optical amplifiers and other optical units.